Prevent selection undo from modifying read-only documents
Prevent selection undo from modifying read-only documents

Closes #5217

namin/CodeMirror,hackmdio/CodeMirror,jkaplon/CodeMirror,wingify/CodeMirror,jkaplon/CodeMirror,notepadqq/CodeMirror,thomasjm/CodeMirror,notepadqq/CodeMirror,ficristo/CodeMirror,nightwing/CodeMirror,SidPresse/CodeMirror,wingify/CodeMirror,hackmdio/CodeMirror,SidPresse/CodeMirror,codio/CodeMirror,pabloferz/CodeMirror,jkaplon/CodeMirror,notepadqq/CodeMirror,nightwing/CodeMirror,ficristo/CodeMirror,hackmdio/CodeMirror,pabloferz/CodeMirror,namin/CodeMirror,pabloferz/CodeMirror,dperetti/CodeMirror,mtaran-google/CodeMirror,SidPresse/CodeMirror,mtaran-google/CodeMirror,thomasjm/CodeMirror,dperetti/CodeMirror,dperetti/CodeMirror,mtaran-google/CodeMirror,namin/CodeMirror,codio/CodeMirror,thomasjm/CodeMirror,wingify/CodeMirror,ficristo/CodeMirror,nightwing/CodeMirror,codio/CodeMirror